Example: bridging Ethernet segments using tinc under Linux

Normally, in the default router mode, tinc will only tunnel IPv4 and IPv6
unicast packets. However, since 1.0pre5 there is an option to let the tinc
daemon act as a switch or a hub (using the Mode configuration variable). This
mode is necessary for tinc to pass non-IP based protocols (NetBEUI, AppleTalk,
IPX, etcetera), and to allow broadcast-based functionality in some applications
(Windows ‘Network Neighborhood’ without a WINS server, among others) to be
usable on a VPN created with tinc.

In switch and hub mode, broadcast packets are broadcast to other daemons and
(in switch mode) MAC addresses are dynamically learned from other tinc daemons
in order to route packets. With these mode tinc can be used to act as a bridge
between two or more Ethernet segments.

Bridging allows all nodes in the VPN to share the same subnet. However, if
this is the only reason for bridging, and you do not need to tunnel broadcast
or non-IP packets, you can alternatively use proxy ARP
instead of bridging.

Overview

The network setup is as follows:

Internal network, on both sides, is 192.168.0.0/16

The host’s own IP address on the internal network is 192.168.10.20

The gateway of each segment has an external interface, eth0, and an internal
interface eth1. Furthermore a bridge interface will be created with name
“bridge”, and the internal interface will be made a slave of this bridge. The
virtual network interface used by tinc will also be a slave. Configuration of
the kernel In addition to the standard kernel configuration described in the
Configuring the kernel section of the manual, a bridge device needs to be added
to your kernel configuration.

To add the bridge device to the Linux 2.4.0 and higher kernels, select the
option under ‘Networking options’ called 802.1d Ethernet Bridging. You may
either compile this option as a module or build it into the kernel.
Configuration of the interfaces Switch and hub modes require that both sides of
a tinc VPN be contained within the same subnet (in this example, the subnet is
192.168.0.0/16). This is no different from the configuration that would be
required if tinc was replaced with an actual switch or hub.

Configuration of tinc

Note that switch and hub mode do not utilize the Subnet variable in the host
files. Instead, any packet received by the bridge interface will be passed to
the TUN/TAP device for processing. If your tinc instance is running in hub
mode, all packets are forwarded to the remote tinc instance. In switch mode,
tinc maintains an ARP cache to determine whether any received packet should be
forwarded to the remote tinc instance.

Additional Configuration

If the Ethernet interface added to the bridge was used for the default route,
you will need to re-add the default route.

If you want to be able to filter packets on your bridge interface, you will
need to a kernel with ebtables support.
More information For more information on Linux bridging, see the bridge-utils
homepage.